Multimodal Ice Crystal Size Distributions in Atlantic Coast Snowstorms: Results from IMPACTS

In January and February 2020, 2022, and 2023, the Investigation of Microphysics and Precipitation for Atlantic Coast Threatening Snowstorms (IMPACTS) was conducted out of Wallops Island, Virginia to study the banding of precipitation in snowstorms. The NASA Airborne Science Program’s P-3 Orion (N426NA) research aircraft flew inside snow bands between −30°C and 0°C to study the microphysics of these bands. Context of the in-situ cloud particle measurements is provided by both WSR-88D (NEXRAD) radars and the NASA ER-2 (809, 806) aircraft which utilized a High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) and Doppler radar (EXRAD) and flew coordinated with the P-3 on many flights. Installed on the P-3 were a Water Isotope System for Precipitation and Entrainment Research (WISPER) probe measuring total water content (TWC), a King probe measuring liquid water content (LWC), a turbulent air motion measurement system (TAMMS) measuring temperature and vertical velocity, a Rosemount icing detector (RICE) identifying supercooled water presence, a Cloud Droplet Probe (CDP) measuring cloud droplet size distributions, a Two-Dimensional Stereo (2D-S) probe and a High Volume Precipitation Spectrometer (HVPS-3) measuring particle and habit size distributions (PSDs), and a Particle Habit Imaging and Polar Scattering (PHIPS) probe acquiring high-resolution particle images and scattering functions. A phase identification algorithm using data from all of the probes is used to classify each particle at temperatures T < 0°C as ice or supercooled liquid water, calculate the m-D relation for each ice particle habit, and characterize each 1 s time period in flight as ice-phase, liquid-phase, or mixed-phase. For ice PSDs in ice- and mixed-phase clouds, a previously developed fitting routine that automatically determines whether a unimodal, bimodal, or trimodal gamma distribution best fits an observed PSD is used to determine the characteristics of PSDs (e.g., presence of unimodal, bimodal, or trimodal distributions, and gamma fit parameters of each of these distributions characterized by volume of equally realizable parameters to account for variability and uncertainty). The dependence of the derived fit parameters and bulk properties (median mass diameter, IWC) on environmental conditions (temperature, IWC, vertical velocity, storm location, etc.) is characterized and compared against those obtained in previous studies characterizing winter snowstorms and other meteorological conditions. Bimodal PSDs with maximum dimension D > 1 mm separating the modes were more common in convective regions than in stratiform regions. Unimodal gamma fit parameters decreased with median mass diameter, varied minimally with IWC, and varied more between and within temperature layers in convective regions than in stratiform regions, with no significant correlation between gamma fit parameters and T in convective regions. Median mass diameters were larger in convective regions than in stratiform regions and increased with T and IWC. The implications for the understanding of processes occurring in snow bands are discussed.